1. Field of the art
This invention relates to novel propylene-based copolymers which are highly suitable for forming films having excellent heat-sealing characteristics, transparency or luminous transmittance, and blocking resistance.
Films made of polypropylenes have excellent mechanical, optical, or thermal characteristics and, for this reason, are being widely used as materials for packaging of commodities such as foodstuffs and textile products. For imparting to the polypropylene films the forms of containers such as bags in this case, or for tightly sealing these containers after placing of the commodities therein, the heat-sealing characteristics (good or poor) have a great influence on the packaging speed, rate of incidence of articles with incomplete seals, deterioration of the packaged commodity due to the sealing heat, and other results.
Therefore, there is an extremely urgent demand for improvement of the heat-sealing characteristics of polypropylene films in cases where these films are to be used as single-layer films and also in cases where they are to be used as composite films each having on its surface an easily heat-sealable layer. Furthermore, it is known that the heat-sealing characteristics of a film are closely related to the melting/crystallizing characteristics of the resin from which that film has been made, and that the lower the melting temperature or the crystallizing temperature, the lower is the heat-sealing temperature at which a specific seal strength can be obtained.
On the one hand, among the various characteristics required of a packaging film, blocking resistance, for example, ordinarily has the tendency of deteriorating with an improvement in the heat-sealing properties, and the question of how the heat-sealing characteristic can be improved without impairing the blocking resistance becomes a crux of the problem of advancing the art.
A further consideration is that it is necessary for a film to be amply transparent for its use as a packaging film, and an improvement in the heat-sealing properties which entails a sacrificing of transparency or luminous transmittance is of low value.
2. Prior art
As polypropylene-based resins for readily heat-sealing films, numerous crystalline random copolymers of propylene and ethylene have been proposed in the prior art.
For example, in Japanese Patent Publication Nos. 31478/1971 and 14343/1974, examples of copolymers of propylene and ethylene containing at least 75 percent by weight of propylene and crystalline random copolymers of propylene and ethylene containing 1 to 6 percent by weight of ethylene are set forth. However, the heat-sealing temperatures of these copolymers to be applied in order to obtain heat-sealing strengths necessary for practical use could not be said to have been amply lowered.
Furthermore, as the ethylene content is increased, the heat-sealing properties are undoubtedly improved, albeit to a small extent, but the blocking resistance rapidly deteriorates, whereby a polymer of this nature is not suited for practical use. Moreover, the transparency also deteriorates. Further problems arise in the production of these resins. The properties of a copolymer having such a high ethylene content in the polymerization reactor such as for example the particle shapes deteriorate, or the viscosity of the polymer slurry is excessively high, whereby it is difficult to operate the polymerization reactor in a stable manner. As an ultimate result, industrial production becomes impossible.
In Japanese Patent Publication No. 30434/1977, it is stated that a copolymer of propylene and an .alpha.-olefin having 4 to 10 carbon atoms which has a propylene content based on the copolymer of 80 to 95 percent by weight, particularly a copolymer in which butene-1 has been used for the .alpha.-olefin, is optimal as a readily heat sealable layer. As a result of our own tests, also, in a high-content region above 10 percent by weight of the butene-1 content in a copolymer of propylene and butene-1, the heat-sealing properties are certainly improved. Nevertheless, the heat-sealing temperature for obtaining a seal strength of 500 g per 20-mm width by our method as described hereinafter, is of the order of 130.degree. to 135.degree. C., and the transparency or luminous transmittance also appears to be the same as or somewhat inferior to that of a propylene-ethylene copolymer.
In Japanese Patent Laid-Open Publication No. 11281/1977, it is stated that good results can be obtained by using as an easily heat sealable layer a crystalline random copolymer having a composition of 86 to 98.9 percent by weight of propylene, 1 to 10 percent by weight of a straight-chain .alpha.-olefin other than propylene, and 0.1 to 4.0 percent by weight of ethylene. However, according to the results of the examples given therein and our own tests, this prior invention appears to provide a film of a heat-sealing temperature of the order of 130.degree. C. As a result of our supplementary tests, we have confirmed that, as the ethylene and .alpha.-olefin contents are increased within the ranges set forth in the above publication in order to lower the lower-limit temperature of the sealable temperature, there occurs a serious deterioration of the properties of the copolymer particles, and stable operation of the polymerization reactor becomes impossible. In addition, the product film undergoes severe blocking.